A deep-sea basin-plain turbidite succession of Ordovician to Devonian age is exposed between 25°30' and 27°S in the coastal region of northern Chile. In the S these turbidites form a large-scale tectonic mélange. Alkali basalt pillow lavas and tuffs are interstratified with the sediments to the S of 26°20'S. The rocks were metamorphosed to the greenschist facies and subjected to two phases of deformation subsequent to the formation of the mélange. The first phase is characterized by large-scale, tight chevron folds and an axial planar cleavage. The folds of the second phase are smaller and more upright and accompanied by a crenulation cleavage. The fold axes of both phases trend between W–E and NW–SE, oblique to the N–S trending Mesozoic and Cenozoic Andean structures. Folds are predominantly asymmetric and overturned towards the S and SW, suggesting an origin by underthrusting towards the N and NE. The deformed succession is interpreted as a subduction complex produced by oblique underthrusting of an oceanic plate towards the N and NE during late Devonian to Carboniferous times.

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